1. Field of the Invention
The present invention relates to line drivers and, more particularly, to a calibrated line driver.
2. Description of the Related Art
A line driver is a device that drives a signal onto a transmission line, such as a local-area-network or telephone line. Line drivers are typically associated with transmit protocols that define the characteristics of the signal that is driven onto the line.
FIG. 1 shows a schematic diagram that illustrates a conventional line driver 100. As shown in FIG. 1, driver 100 includes a transmit circuit 110 which has a pair of differential outputs OUT1+ and OUT1-, and a transformer 112 which has a pair of inputs IN+ and IN- that are connected to the outputs OUT1+ and OUT1-. In addition, transformer 112 also has a pair of outputs OUT2+ and OUT2- that are connected to a transmission line 114, such as a CAT-5 coaxial cable.
Transmit circuit 110 can be implemented as a current-based circuit or as a voltage-based circuit. A current-based circuit can be implemented in a variety of ways, but typically includes a number of resistors, a number of current sources, and a number of switches.
FIG. 2A shows a schematic diagram that illustrates a first example of a current-based transmit circuit 110. As shown in FIG. 2A, circuit 110 includes a resistor R which is formed across the inputs IN+ and IN- of transformer 112, a first switch S1 which is connected between a power supply voltage Vcc and the input IN+, and a first current source 210 which is connected between the input IN- and ground.
In addition, circuit 110 also includes a second switch S2 which is connected between the power supply voltage Vcc and the input IN-, and a second current source 212 which is connected between the input IN+ and ground. Further, circuit 110 also includes a control circuit 214 that controls the operation of switches S1 and S2.
In operation, when switch S1 is closed and switch S2 is open, current source 210 pulls a current I.sub.p, through resistor R which sets up a positive output voltage V.sub.OD1 across the inputs IN+ and IN- of transformer 112. On the other hand, when switch S1 is open and switch S2 is closed, current source 212 pulls a current I.sub.N through resistor R which sets up a negative output voltage V.sub.OD2 across the inputs IN+ and IN- of transformer 112. As shown, the negative output voltage V.sub.OD2 has a polarity opposite to the polarity of voltage V.sub.OD1.
In addition, when switches S1 and S2 are both open, a voltage intermediate to the output voltages V.sub.OD1 and V.sub.OD2 is placed across the inputs IN+ and IN- of transformer 112. (An intermediate voltage is required by some transmission protocols, such as MLT3.)
FIG. 2B shows a schematic diagram that illustrates a second example of a current-based transmit circuit 110. Circuit 110 shown in FIG. 2B is similar to circuit 110 shown in FIG. 2A and, as a result, utilizes the same reference numbers to designate the structures which are common to both figures.
Circuit 110 of FIG. 2B differs from circuit 110 of FIG. 2A in that a third switch S3 and a multiplying digital-to-analog converter (DAC) 216 are used in lieu of current sources 210 and 212. Third switch S3, which is controlled by control circuit 214, has first and second positions P1 and P2. DAC 216, in turn, receives a bandgap current I.sub.BG from a bandgap current source 218, an n-bit control word CW, and sinks a DAC current I.sub.DAC which is defined by the bandgap current I.sub.BG and the control word CW.
Conventionally, switches S1, S2, and S3, DAC 216, and current source 218 are formed as part of a transmit integrated circuit, while resistor R is externally connected to the transmit integrated circuit. Control circuit 214, in turn, can be part of the transmit integrated circuit, or part of another integrated circuit that outputs control signals to the transmit integrated circuit.
In operation, when switch S1 is closed, switch S2 is open, and switch S3 is in position P1, DAC 216 pulls DAC current I.sub.DAC through resistor R which sets up the voltage V.sub.OD1 across the inputs IN+ and IN- of transformer 112.
On the other hand, when switch S1 is open, switch S2 is closed, and switch S3 is in position P2, DAC 216 pulls DAC current I.sub.DAC through resistor R which sets up the voltage V.sub.OD2 across the inputs IN+ and IN- of transformer 112.
As above, voltage V.sub.OD2 has a polarity which is opposite to the polarity of voltage V.sub.OD1. In addition, when switches S1 and S2 are both open, a voltage intermediate to the output voltages V.sub.OD1 and V.sub.OD2 is placed across the inputs IN+ and IN- of transformer 112.
FIG. 3A shows a schematic diagram that illustrates a third example of a current-based transmit circuit 110. As shown in FIG. 3A, circuit 110 includes a first resistor R1 which is connected between the input IN- and a power supply voltage Vcc, and a second resistor R2 which is connected between the input IN+ and the power supply voltage Vcc.
As further shown in FIG. 3A, circuit 110 includes a first switch S1 which is connected in parallel with resistor R2, and a second switch S2 which is connected in parallel with resistor R1. In addition, circuit 110 further includes a first current source 310 connected between the input IN- and ground, and a second current source 312 connected between the input IN+ and ground. Further, circuit 110 also includes a control circuit 314 that controls the operation of switches S1 and S2.
In operation, when switch S1 is closed and switch S2 is open, the power supply voltage Vcc is shorted to the input IN+, while current source 310 pulls a current I.sub.p through resistor R1 which sets up a voltage VP on the input IN- which is less than the power supply voltage Vcc-VP is a result, a voltage V.sub.OD1 equal to Vcc-VP is dropped across the inputs IN+ and IN- of transformer 112.
On the other hand, when switch S1 is open and switch S2 is closed, the power supply voltage Vcc is shorted to the input IN-, while current source 312 pulls a current I.sub.N through resistor R2 which sets up a voltage VN on the input IN+ which is less than the power supply voltage Vcc.
As a result, a voltage V.sub.OD2 equal to Vcc-VN is dropped across the inputs IN+ and IN- of transformer 112. In addition, when switches S1 and S2 are both open, a voltage intermediate to the output voltages V.sub.OD1 and V.sub.OD2 is placed across the inputs IN+ and IN- of transformer 112.
FIG. 3B shows a schematic diagram that illustrates a fourth example of a current-based transmit circuit 110. Circuit 110 shown in FIG. 3B is similar to circuit 110 shown in FIG. 3A and, as a result, utilizes the same reference numbers to designate the structures which are common to both figures.
Circuit 110 of FIG. 3B differs from circuit 110 of FIG. 3A in that a third switch S3 which has first and second positions P1 and P2, and a multiplying DAC 316 are used in lieu of current sources 310 and 312. DAC 316 receives a bandgap current I.sub.BG from a bandgap current source 318, an n-bit control word CW, and sinks a current I.sub.DAC which is defined by the bandgap current I.sub.BG and the control word CW.
Conventionally, switches S1, S2, and S3, DAC 316, and current source 318 are formed as part of a transmit integrated circuit, while resistors R1 and R2 are externally connected to the transmit integrated circuit. Control circuit 314, in turn, can be part of the transmit integrated circuit, or part of another integrated circuit that outputs control signals to the transmit integrated circuit.
In operation, when switch S1 is closed, switch S2 is open, and switch S3 is in position P1, the power supply voltage Vcc is shorted to the input IN+, while DAC 316 pulls current I.sub.DAC through resistor R1. The current I.sub.DAC sets up a voltage VP on the input IN- which is less than the power supply voltage Vcc. As a result, the voltage V.sub.OD1 (equal to Vcc-VP) is dropped across the inputs IN+ and IN- of transformer 112.
On the other hand, when switch S1 is open, switch S2 is closed, and switch S3 is in position P2, the power supply voltage Vcc is shorted to the input IN-, while DAC 316 pulls current I.sub.DAC through resistor R2. The current I.sub.DAC sets up the voltage VN on the input IN+ which is less than the power supply voltage Vcc.
As a result, the voltage V.sub.OD2 (equal to Vcc-VN) is dropped across the inputs IN+ and IN- of transformer 112. In addition, when switches S1 and S2 are both open, a voltage intermediate to the output voltages V.sub.OD1 and V.sub.OD2 is placed across the inputs IN+ and IN- of transformer 112.
FIG. 4A shows a schematic diagram that illustrates a fifth example of a current-based transmit circuit 110. As shown in FIG. 4A, circuit 110 includes a first resistor R1 which is connected between the input IN- and a power supply voltage Vcc, and a second resistor R2 which is connected between the input IN+ and the power supply voltage Vcc.
As further shown in FIG. 4A, circuit 110 includes a first switch S1 which is connected to resistor R1 and the input IN-, and a second switch S2 which is connected to resistor R2 and the input IN+. In addition, circuit 110 further includes a first current source 410 which is connected between switch S1 and ground, and a second current source 412 which is connected between the input IN+and ground. Further, circuit 110 also includes a control circuit 414 that controls the operation of switches S1 and S2, and a transformer 416 which has a center tap connected to the power supply voltage Vcc.
In operation, when switch S1 is closed and switch S2 is open, the power supply voltage Vcc is present on the input IN+ as no current flows through resistor R2, while current source 410 pulls a current I.sub.P through resistor R1 which sets up a voltage VP on the input IN- which is less than the power supply voltage Vcc. As a result, a voltage V.sub.OD1 equal to Vcc-VP is dropped across the center tap and the input IN- of transformer 112.
On the other hand, when switch S1 is open and switch S2 is closed, the power supply voltage Vcc is present on the input IN- as no current flows through resistor R1, while current source 412 pulls a current I.sub.N through resistor R2 which sets up a voltage VN on the input IN+ which is less than the power supply voltage Vcc. As a result, a voltage V.sub.OD2 equal to Vcc-VN is dropped across the center tap and the input IN+ of transformer 112.
FIG. 4B shows a schematic diagram that illustrates a sixth example of a current-based transmit circuit 110. Circuit 110 shown in FIG. 4B is similar to circuit 110 shown in FIG. 4A and, as a result, utilizes the same reference numbers to designate the structures which are common to both figures.
Circuit 110 of FIG. 4B differs from circuit 110 of FIG. 4A in that a third switch S3 which has first and second positions P1 and P2, and a multiplying DAC 418 are used in lieu of current sources 410 and 412. DAC 418 receives a bandgap current I.sub.BG from a bandgap current source 420, an n-bit control word CW, and sinks a current I.sub.DAC which is defined by the bandgap current I.sub.BG and the control word CW.
Conventionally, switches S1, S2, and S3, DAC 418, and current source 420 are formed as part of a transmit integrated circuit, while resistors R1 and R2 are externally connected to the transmit integrated circuit. Control circuit 414, in turn, can be part of the transmit integrated circuit, or part of another integrated circuit that outputs control signals to the transmit integrated circuit.
In operation, when switch S1 is closed, switch S2 is open, and switch S3 is in position P1, the power supply voltage Vcc is present on the input IN+ as no current flows through resistor R2, while DAC 418 pulls current I.sub.DAC through resistor R1. The current I.sub.DAC sets up the voltage VP on the input IN- which is less than the power supply voltage Vcc. As a result, the voltage V.sub.OD1 (equal to Vcc-VP) is dropped across the inputs IN+ and IN- of transformer 112.
On the other hand, when switch S1 is open, switch S2 is closed, and switch S3 is in position P2, the power supply voltage Vcc is present on the input IN- as no current flows through resistor R1, while DAC 316 pulls current I.sub.DAC through resistor R2. The current I.sub.DAC sets up the voltage VN on the input IN+ which is less than the power supply voltage Vcc. As a result, the voltage V.sub.OD2 (equal to Vcc-VN) is dropped across the inputs IN+ and IN- of transformer 112.
FIG. 5 shows a schematic diagram that illustrates an example of a voltage-based transmit circuit 110. As shown in FIG. 5, circuit 110 includes a first DAC 510, and a first bandgap-derived current source 512 that supplies a first biasing current I.sub.BG1 to DAC 510. DAC 510 receives an n-bit control word CW1, and outputs a DAC current I.sub.DAC1 in response to the biasing current I.sub.BG1 and the control word CW1.
Circuit 110 also includes a second DAC 514, and a second bandgap-derived current source 516 that supplies a second biasing current I.sub.BG2 to DAC 514. DAC 514 receives a m-bit control word CW2, and outputs a DAC current I.sub.DAC2 in response to the biasing current I.sub.BG2 and the control word CW2. (The n and m values may be equal.)
As further shown in FIG. 5, circuit 110 includes a first resistor R1 which is connected between the output of DAC 510 and ground, and a second resistor R2 which is connected between the output of DAC 514 and ground.
Circuit 110 further includes a first operational amplifier (op amp) 520 which has a non-inverting input connected to the output of DAC 510, and an inverting input connected to the output of op amp 520. Further, a third resistor R3 is connected between the output of op amp 520 and the input IN+ of transformer 112.
Circuit 110 additionally includes a second op amp 522 which has a non-inverting input connected to the output of DAC 514, and an inverting input connected to the output of op amp 522. A fourth resistor R4 is connected between the output of op amp 522 and the input IN- of transformer 112. Further, a control circuit 524 is connected to supply the control words CW1 and CW2.
Conventionally, DACs 510 and 514, current sources 512 and 516, and op amps 520 and 522 are formed as part of a transmit integrated circuit, while resistors R1, R2, R3, and R4 are externally connected to the transmit integrated circuit. Control circuit 524, in turn, can be part of the transmit integrated circuit, or part of another integrated circuit that outputs control signals to the transmit integrated circuit.
In operation, when the control words CW1 and CW2 cause DAC 510 to output a greater current than DAC 514, a positive output voltage V.sub.OD1 is placed across the inputs IN+ and IN- of transformer 112. Similarly, when the control words CW1 and CW2 cause DAC 510 to output a lesser current than DAC 514, a negative output voltage V.sub.OD2 is placed across the inputs IN+ and IN- of transformer 112.
When the control words CW1 and CW2 cause the DAC currents I.sub.DAC1 and I.sub.DAC2 to be equal, op amps 520 and 522 output equal but opposite voltages which, in turn, cause a voltage intermediate to the maximum output voltages V.sub.OD1 and V.sub.OD2 to be placed across the inputs IN+ and IN- of transformer 112.
Regardless of which transmit circuit is utilized, the transmit protocols for multiple-port transmit circuits typically require the ports to output matching differential output voltages V.sub.OD when presented with equivalent input conditions.
In actual practice, this is a difficult condition to meet. Initially, it is difficult to obtain matching output voltages V.sub.OD at a reference temperature, such as 50.degree. C., because subtle variations in the internal routing within the transmit circuit can lead to unintended voltage drops which, in turn, lead to mismatched output voltages V.sub.OD.
Once this hurdle is cleared, the differential voltage V.sub.OD output from each port typically varies at a different rate with variations in temperature. Further, variations in the manufacturing process can lead to variations in the output voltage V.sub.OD.
These differences further increase the problem because it is difficult to make circuits that track process and temperature well. Thus, there is a need for a circuit that accounts for variations in the output voltages V.sub.OD to provide matched output voltages V.sub.OD in multiport drivers.